This invention relates to a prosthetic femoral joint which comprises a part-spherical cup adapted for location in an acetabulum and having a part-spherical inner bearing surface to receive a part-spherical ball head which can be attached to a stem for location in a femur and in which the inner bearing surface of the cup extends around an angle of more than 180°. Cups of this type are known which comprise a single element made, for example, of a synthetic plastics material or metal. The inner bearing surface can also be formed on an insert which again can be of a synthetic plastics material, a ceramic, or a metal and which is carried in an outer housing or shell, the housing engaging the acetabulum with which it is to be used and being held in place by, for example, cement or by mechanical means, for example nails or screws. The cup is used in connection with Total Hip Arthroplasty which includes implanting a femoral component in the femur which component normally includes the ball head.
In other known constructions of the cup the inner bearing surface can be provided on an inner layer or insert made from a different material from an outer backing which engages the acetabulum. Dual mobility cups or bipolar cups generally comprise an inner bearing surface which receives the part-spherical ball head and which is itself freely rotably mounted in a part-spherical element which has outer and inner bearing surfaces. The outer bearing surface engages the inner bearing surface of an outer housing which engages the acetabulum. This type of cup allows greater mobility of angular movement. Such cups are shown in U.S. Pat. No. 4,798,610 and U.S. Publication No. 2004/0143341.
In all these constructions, and, more especially, dual mobility cups, it is advantageous to have a means for retaining the ball head against the inner bearing surface. There are various ways of achieving this with a hard material and at least two ways of retaining the ball head inside the inner bearing surface. For example, U.S. Pat. No. 4,798,610 uses a ring seated on a conically tapered surface in the inner bearing. A deformable material cannot be used and in a first arrangement a ring or circlip is used but this requires a third part and there is risk of wear. The second arrangement can consist of having two flats on the head and turning the head at 90° prior to inserting it into the cup and then again turning the head back through 90°. A disadvantage with this type of construction is that specific heads are required and there is the risk of wear due to the truncating of the head. A system of this type is shown in FR 2 785 525 and FR 2 807 315.
The present invention is intended to overcome some of the disadvantages referred to above.